CIS 338: VB Control Structures

Dr. Ralph D. WestfallJanuary, 2006

Computer Programming Words

statements = instructions to computercode blocks lines of statements that work together

execution = running program statementsevaluation = determining values calculating and/or substituting within

statement If A = B Then … substitute actual values for A and B,

then test to see if values are equal

Computer Program Words - 2

flow: order in which statements execute action statements do the actual work control statements determine flow control structure = control statements

that go with each other, plus action statements

'comments are ignored by computer

Another Computer Program Word: Concatenation

“link together in series or chain” (Webster)

& concatenates items regardless of type "Le " & "Dinh" = "Le Dinh" (note space) 1 & 1 = "11"

+ concatenates strings like &, but: 1 + "1" = 2

Common Programming Tasks

handle items differently based on a comparison admission prices based on ages

do very same thing with different items send same letter to 400 students

use same code for different processes saving Word, Excel, or PowerPoint

files to C:\ drive

Selection/Branching - If

always based on a condition (test) condition should evaluate to True or

False A = B A > B AND C < D True

if evaluates to 0, becomes False, e.g., 1-1

one-line If statement If [condition] Then [code] 'don't forget the Then 'code runs only if True

Selection/Branching - If 2

“block” If statement If [condition] Then [line(s) that run when True] End If 'not EndIf (must put space 'between End and If)

Selection/Branching - If 3

If [condition] Then [code that runs if True] Else [code that runs if False] End If

Selection/Branching - If 4

If [condition 1] Then [code runs if 1 True] ElseIf [condition 2] Then [code runs if 2 True] Else 'this/next line optional [code runs if both False] End If 'ElseIf, not Else If

Selection/Branching - If 5

“nested” If statements (indents clarify)

If [condition 1] Then [code runs if 1 True] If [condition 2] Then [code runs if 1 & 2 True] End If 'end of 2nd If End If 'end of 1st If 'example? clothes: gender & income

Selection/Branching - Case

use Select Case for multiple tests makes code easier to read and maintain

starts with Select Case [variable name] 'value can have text or Boolean as well as

numeric values (contra Java)includes multiple variable/action pairs Case [criterion] ' (value or variable) [code to run if variable matches criterion]

may have Case Else (and code) after other pairsends with End Select 'not EndSelect

Selection/Branching - Case 2

Select Case sVegetable 'NG in Java

Case "broccoli" CaloryCount = 50 Case "corn" CaloryCount = 111 Case "peas" CaloryCount = 77 Case Else CaloryCount = 925 End Select

Selection/Branching - Case 3

Case can use other types of selections

Select Case [variable or formula]Case 4, 7, 9 'list

[code to run]Case 11 To 20 ' range

[code to run]Case Is >= 30 'logical comparison

[code to run]

Selection/Branching - Case 4

Comparing to a logical value (like Case Is) Select Case True (or expression) Case nMPG < 10 'evaluated sCategory = "gas guzzler" Case sColor = "blue" sCategory = "dull" Case nTopSpeed > 130 OR sMfg _ = "Ferrari"

sCategory = "sporty" End Select 'Is not used in compare

Selection/Branching - Case 5

usually should have a Case Else tests always stop after a match is found put most common matches near top DON'T let test values overlap like below

Select Case nAge Case Is < 5 strSchool = "preschool" Case Is < 18 strSchool = "highschool" Case Is < 14 'will never run strSchool = "elementary"

Loop Structures

use when need repeated actionsFor…Next know how many repetitions

e.g. process 10 items in an array can use variable for number of

repetitions

Do While or Do Until keep looping until something happens e.g. keep processing to end of file

Loop Structures - For/Next

For nIndex = nStart to nEnd [code to run]

Next nIndex '2nd word is optionalnIndex is an integer variable can access its value in the code in the loop value increments by 1 after each repetition

nStart & nEnd are variables, or integer #s loop ends after loop runs code for last time

(here: last time is when nIndex = nEnd)

Enumeration Loop - For/Each

Dim decItem as Decimal Dim decProdPrice(1) as Decimal decProdPrice(0) = 5.02D 'zero based decProdPrice(1) = 15.99D

For Each decItem in decProdPrice MsgBox(decItem)

Next decItem 'non-integer OK as Eachprocesses all members of an array don't have to know how many members works somewhat like a Do Until

Loop Structures - Do While

Dim nCalc as IntegernCalculated = CInt(Text1.Text)Do While nCalc < 100 nCalculated = Math.Abs(nCalc * 2)Loop 'Abs handles < 0 caseText1.Text = CStr(nCalculated)

keeps going until test evaluates to Falseif test is False at start, loop won't run e.g., if user's initial input is >= 100

Loop Structures – Do… Loop Until

'an initial ReadLine is before loopDo intDataItem = CInt(strInputLine) intTotal += intDataItem strInputLine = strDataInputs.ReadLine() Loop Until strInputLine Is Nothing

keeps going until test evaluates to Trueeven if test is True at start, the block will still run one time

Loops – Do ... Loop While

Dim nTry as Integer Dim nAnswer as Integer = 5 Do nTry = CInt(InputBox("Guess #")) Loop While nTry <> nAnswer

keeps going until test = Falsewill always run code at least one time can also use Do ... Loop Until 'by changing what?

Transfer of Control (“Jumping”)

Subprocedure = block of code that is used often in different parts of a program doesn't return a value often sets value(s) in other parts of

program

Function is similar to a subprocedure but it returns a value to the segment of

code that calls it

Subprocedure (Subroutine)

Sub [name]([By[ ] [args as types]) Handles …

'can include 1+ args after [name][1 or more lines of code to run]

End Sub

can be called (run) in two waysCall [name (args)] 'OR[name (args)] 'the word Call is optional (but I like it)

Subprocedure - 2

Sub CopyRightInfo(ByVal nYear as _ Integer) Handles …

strCRight = VBCRLF & _ "© R. Westfall " & nYear & _ VBCRLF & "All rights reserved"

End Sub

[lines of code] Call CopyRightInfo (2001)

[lines of code] CopyRightInfo(1999)

Scope Considerations

Sub Calc(ByVal nA as Integer, ByVal nB _ as Integer, ByVal nC as Integer)

nC = nA + nB 'nC has local scopeEnd Sub

Call Calc(nNum1, nNum2, nNum3) nTotal = nC 'won't work on local variablenTotal = nNum3 'works OK

Call Calc(nNum1, nNum2, nTotal)'better: don't need nTotal = nNum3 line

Pass by: Reference vs. Value

Call Calc(nNum1, nNum2, nTotal)'passes values "by value" (default)'sub uses copied values of these arguments 'a change of its value in the sub doesn't 'change the value of the argument in the 'calling procedure

Sub Calc(ByRef a as Integer)'changing a in Sub changes value

elsewhere

Reference vs. Value - 2

Sub Calc(ByRef nA as Integer,_ ByVal nB as Integer, ByVal _

nC as Integer)

'nB, nC passed by value'nB and nC values will not be changed

in'calling procedure, even if changed in

sub

Function

Function [fName]([args]) As [type])' can include argument(s) after [name]' argument(s) & function have data type' declarations

[1 or more lines of code to run][fName] = [some value or calculation]'1 line can set value of [function name]

by' having name of function to left of =

End Function

Function - 2

Function SevenX(ByVal i as Integer) As _ Integer 'declare return value

SevenX = i * 7 'function name

'Return i * 7' Return unnecessary if use function name

as' variable to hold calculated return value

End Function

Function - 3

call function by its name in an equation dTotal = CalcTotal(nVisitors,

dPayment)

value is returned in function name function name above is CalcTotal

when you write [function name(args)], the function returns the value it calculates

Function - 4 Function ListPrice(fCost As Single) As Single 'arg type result type

ListPrice = fCost * 1.5 End Function

[lines of code] fPrice(1) = ListPrice(fDealerCost(1)) fPrice(2) = ListPrice(fDealerCost(2) * .95)

'parameter name does not have to be same here 'as in the function

'array variables here, regular variables in 'function (evaluated and substituted)

'can also do calculations within arguments

Subroutine/Function Issues

functions & subroutines used in multiple forms could go into a Class or Modulecan pass objects to subroutines and functions

Sub SetValues(txtName As Object) txtName.Text = "This doesn't work"txtName.Enabled = False

End Sub 'txtName is a TextBox

Subprocedure and Function Advantages

saves typing (or pasting) can use subprocedure or function

name rather than keep repeating same block of code

makes code easier to read and maintain smaller programs code is organized into blocks

Subprocedure and Function Disadvantages

extra work to make code more general may need to pass Boolean variables

to indicate special processing for certain types of itemse.g., to indicate different/extra

processing for different types of customers

extra work may offset the benefits